As world population and industrial development have increased in the face of progressively stricter regulation and enforcement of environmental standards, substantial work has been directed toward effective processes for purifying soil and water polluted by organic chemicals. Some of these processes are aimed at cleaning up soils and water contaminated by prior discharges of waste organic compounds from industry and agriculture. Other processes are aimed at minimizing further release of such compounds into the environment. Many such processes are ineffective because a number of organic compounds, especially various halogenated species, are refractory, being resistant to biological or chemical attack by existing means or unless excessive amounts of energy are expended. Unfortunately, many halogenated organic compounds are toxic; some are known or suspect carcinogens or mutagens. Hence, as these compounds become more widespread in soil and water around the world, the need for efficacious and inexpensive methods for treating such wastes becomes increasingly urgent.
Halogenated organic compounds can be separated from aqueous liquids by conventional technology. However, the process is expensive and still results in a complex mixture of halogenated compounds that must be purified from one another to be of any practical use. Unless the constituent compounds can be repurified for further industrial use, such mixtures of halogenated organic compounds remain a waste material that presents a serious disposal problem.
Some existing processes have employed various types of microorganisms to degrade the pollutants biologically. For example, U.S. Pat. No. 4,401,569 to Jhaveri et al. discloses a method and apparatus for treating ground water contaminated with certain halogenated hydrocarbons. In the Jhaveri process, ground water is removed and stored in a holding tank from which the water is delivered to a biostimulation tank. The biostimulation tank contains microorganisms naturally occurring at the contamination site or introduced thereto, which utilize the particular organic pollutant(s) as a source of carbon and energy. Nutrients, including certain inorganic salts and other unspecified nutrients, are added to the biostimulation tank to accelerate the organisms' metabolism of the pollutants. The biostimulation tank is also aerated with oxygen and/or other unspecified gas. Over time, biodegradation processes decrease the concentrations of organic pollutants in the water in the biostimulation tank. After a length of time, the treated water is transferred to a settling tank where further nutrients are then added to the treated water and from which the water is returned to the soil at the contamination site. Oxygen and/or other unspecified gases are also injected into the soil at the return site. Thus, the Jhaveri et al. patent is primarily concerned with a batch process for the biological removal of certain hydrocarbons from contaminated soil and ground water. Jhaveri et al. do not disclose or suggest supporting methylotrophic microorganisms on a solid substrate bed through which an aqueous mixture containing organic compounds is passed for biodegrading the compounds.
U.S. Pat. No. 4,713,343 of Wilson, Jr. et al. relates to a process for aerobic biodegradation of certain low-molecular-weight halogenated aliphatic hydrocarbons in water using methanotrophic bacteria. The Wilson, Jr. et al. patent is purportedly applicable to the treatment of contaminated drinking water, ground water and industrial wastewater. In the Wilson, Jr. et al. process, methanotrophic bacteria present in soil and water are exposed to oxygen or air and a low concentration of a low-molecular-weight alkane, such as methane or natural gas. Wilson, Jr. et al. mention that the process can occur in any material that can be colonized by alkane-oxidizing bacteria. In one approach, natural gas and air are dissolved in water containing the bacteria and a specific halogenated aliphatic hydrocarbon. After a time, the bacteria degrade the hydrocarbon in the water. Wilson, Jr. et al. specifically disclose a batch process of removing ground water from a site having contaminated soil, treating the water in the above manner, and returning the water to the ground. They also disclose an in situ approach wherein water containing dissolved air and the low-molecular-weight alkane is injected deep into contaminated soil to stimulate indigenous bacteria to degrade the soil contaminant.
Wilson, Jr. et al. describe quantitative results obtained only with laboratory-scale mockups of the in situ approach wherein soil was packed in a glass column to a depth of 150 cm. A stream of air containing 0.6 percent natural gas by volume was passed over the head of the column. Following a three-week acclimation, water containing trichloroethylene was applied to the column at the rate of 21 cm.sup.3 per day. Most of the trichloroethylene in the water was biodegraded.
U.S. Pat. No. 4,385,121 of Knowlton also discloses the use of soil microorganisms to biodegrade hydrocarbon contaminants in the soil. The Knowlton patent describes a land-farming process in which at least one of a spent, solid, particulate porous, hydrocarbon cracking catalyst or a spent, solid, porous particulate filtration medium is tilled or otherwise incorporated into soil contaminated with hydrocarbon wastes. The microorganisms in the soil then biodegrade the waste hydrocarbons. The addition of catalyst or filtration medium improves aeration necessary for supporting microorganism metabolism.
Hence, although prior art approaches are known, a need exists for an improved method and apparatus for biodegrading organic compounds in aqueous mixtures, especially for dehalogenating and further biodegrading recalcitrant halogenated organic wastes, including chlorinated organic compounds, as they are generated by industrial plants.